Apparatus for taking impressions of surface patterns



Nov. 10, 1970 w. E. EDER 3,539,766

APPARATUS FOR TAKING IMPRESSIONS OF SURFACE PATTERNS Filed Oct. 11, 1968 v 3 Sheets-Sheet 1 3,539,766 APPARATUS FOR TAKING IMPRESSIONS OF SURFACE PATTERNS Filed Oct. 11, 1968 w. E. E'DER Nov. 10, 1970 3 sh ets-sheet 2 W. E. EDER APPARATUS FOR TAKING IMPRESSIONS OF SURFACE PATTERNS 3 Sheets-Sheet 3 Filed Oct. 11, 1968 WNQ United States Patent O 3,539,766 APPARATUS FOR TAKING IMPRESSIONS OF SURFACE PATTERNS Wolfgang E. Eder, Alberta, Canada, assignor to United Kingdom Atomic Energy Authority, London, England Filed Oct. 11, 1968, Ser. No. 766,821 Claims priority, application Great Britain, Oct. 18, 1967, 47,415/ 67 Int. Cl. 321 1/06 US. Cl. 219-149 9 Claims ABSTRACT OF THE DISCLOSURE This invention relates to apparatus for taking impressions of physical patterns on surfaces. The invention is particularly though not exclusively applicable for use in a method of structural strain analysis in which a member to be subjected to strain has inscribed in its surface an accurately cut pattern of closely spaced lines or grooves which may typically be in the form of a close spiral. Inspection and analysis of the form of any distortions in the spiral provide accurate information as to the strain imposed on the member.

It is an object of the present invention to provide an improved apparatus for taking a replica or impression of a surface pattern which will be particularly useful in situations where the temperature of the patterned surface may vary appreciably. In a known method of taking an impression of a surface a plate known as a platen is coated with a low melting point material and heated to a temperature close to the melting point of the material, and forced against the surface so that the material while still soft takes a complementary or negative replica of the surface, and then almost instantaneously hardens. It will be appreciated that if the temperature of the surface is subject to variation, or if variations in temperature exist between different structural elements to be investigated, a platen coated with a given material may not be satisfactory. The material selected must of course have a melting point higher than the temperature of the surface but its melting point should not be very much higher and preferably the material and so the melting point should be selected depending upon the actual temperature of the surface at the time of taking the replica. An object of the invention accordingly is to provide apparatus which will facilitate the use of a properly selected material having the appropriate melting point. Preferably the apparatus is also so designed that it is substantially self-contained and portable, and can be used in confined spaces.

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From one aspect the invention consists in apparatus for taking replicas or impressions of physical patterns on surfaces, comprising a body or housing having a mechanical ejection system for ejecting a platen coated with a low melting point material against the surface, means for heating the platen before it is ejected, means sensitive to the temperature of the platen, and a firing device which actuates the mechanical ejection system to discharge the platen when a selected platen temperature is reached.

The apparatus will preferably include means for varying the selected platen temperature at which the firing device is energised, and may also include means for sensing the temperature of the surface carrying the pattern.

In a preferred form of the invention the body or housing is provided with locating elements to engage the said surface and assist in positioning the apparatus correctly, one of these locating elements including a temperature sensing device such as a Thermistor (RTM).

Conveniently the heating device is electrically energised, and preferably the apparatus is portable being adapted for connection to an electric accumulator for energising the heating device and the firing device.

According to another preferred feature of the invention the apparatus includes an automatic sequencing circuit arranged firstly to energise the heating device and secondly to energise the firing circuit when the platen temperature reaches the selected value, and may also include means for checking the state of charge of the accumulator.

From another aspect the invention consists in a method of obtaining a replica of a physical pattern on a surface in which the temperature of the surface is measured, a suitable platen is selected, coated with a low melting point material whose melting point is higher than the sensed surface temperature. The platen is heated to a temperature above the surface temperature and near to the melting point of the material, and is fired against the surface, so that the material hardens and produces a negative replica of the pattern on the surface.

According to another preferred feature of the invention the material of the platen itself is so selected as to have substantially the same coefficient of linear thermal expansion as the material of the surface in question. For eXample a range of platen materials may be provided, including mild steel, aluminium and brass.

The invention also consists in a method of preparing platens for use in such a process, in which a platen is first coated with a thin layer of suitable bonding metal, and the selected low melting point material is then cast on to the pre-coated surface.

An embodiment of the invention will now be described with reference to the accompanying drawings in which:

FIGS. 1, 2 and 3 are perspective views, respectively from the left hand side, from the top, and from the front of an apparatus embodying the invention;

FIG. 4 is a part sectioned view on the section IVIV of FIG. 3; and

FIG. 5 is a circuit diagram.

For the purposes of this example six different low melting point materials (such as alloys) are selected, designed to cover a range of surface temperatures from about 30 C. up to about 140 C. A supply of platens is arranged, each being treated by pre-coating with a layer of one of the selected materials. Each platen comprises a circular metal plate with a diameter of 1.06 inches, having a stem rigidly secured to its reverse face. Typically treatment for pre-coating comprises the face of the platen being cleaned, degreased and then coated with a thin film of a lead-tin solder after which a layer of the selected low melting point material is cast on to the tinned face in a mould. Precoated platens of selected materials and having different material coatings are conveniently supplied for use packed in boxes and marked accordingly.

FIGS. 14 embody apparatus for taking an impression which is in the general form of a pistol and which will hereafter be referred to for convenience as a platen gun 11. The gun '11 comprises a main rectangular housing 12 detachably mounted on a pistol grip 13 having a trigger 14 for operating a firing device as will be hereinafter described.

At the front end of the gun 11 there are provided three spaced supports or legs 15, 1-6, 17 designed to locate and steady the gun against a surface 111 (shown in broken outline). The upper two supports '15, .16 are pointed and the lower support 17 includes (FIG. 3) a temperature sensing element 17a (typically a thermistor). Within the triangle formed by the three supports 15, 16, 17 there is mounted a cylindrical transparent Perspex (R.T.M.) shield or splash guard 18 which is spring loaded so as to be biased forwardly towards the surface. Within the splash guard 18 at the front end of the gun there is provided a heating coil assembly comprising an annular copper heating former 19 on which are wound two separate heating coils. The former 19 has a central aperture 20 to receive the stem 21 of a platen 21a (shown in broken outline) and the front face of the former is formed to make close contact with the rear surface of the platen plate. A further temperature sensing element (not shown) is incorporated in the heating former to engage the rear face of the platen and provide an indication of the platen temperature.

The ejection system for the platen is embodied in a central firing pin 22 (FIG. 4) aligned with the aperture 20 in the heating coil former 19 and arranged to engage the stem of the platen enabling it to be ejected forwardly against the surface 111. This firing pin 22 is actuated by a solenoid 23 connected to mid-point 24 of a swinging lever 25, one end of which is pivotally connected to the firing pin 22 while the other end is pivotally connected to a mounting bracket 26 on the housing 12. A light spring 27 acts rearwardly on the firing pin 22 to return the pin to its initial position after a platen has been fired.

As shown in FIG. 4 the solenoid 23 is in the energised position where firing pin 22 has been driven forward (in the direction of arrow 22a) to eject a platen (not shown). When the solenoid is de-energised spring 27 will act to drive, in the opposite direction to arrow 22a, the pin 22 to a reset position.

The housing 12 has on its upper surface two indicating lamps referred to as the reset and heat" indicators (respectively indicators 28, 29) and also a multipurpose volt meter 30. The rear face of the gun is provided with a three-pin socket 31 for connection to a three-core cable (not shown), leading to a portable accumulator pack (not shown) consisting of nickel-cadmium accumulators giving a fifteen volt supply.

On the left side of the housing 12 (FIG. 1) there is provided a switch 32 for a lamp to illuminate the volt meter, a rotary four-position sequencing switch 33, and two calibrated rotary control knobs of which the upper will be referred to as the surface temperature control knob 34 and the lower as the platen temperature control knob 35. On this side of the gun housing there is also provided a manual firing button 36 which can be used if 4 the pistol grip 13, with its trigger 14, is detached from the housing.

Within the housing 12 there is provided a solid state circuit (FIG. 5) designed to fulfill three main functions. The first function is to check the state of charge of the portable accumulator pack. The second function is to sense the temperature of the surface. The third function is to select the appropriate temperature to which the platen is to be heated and then automatically to eject the platen at the selected temperature and ultimately shut off the current to the heating coils. These three functions can be selected by means of the rotary sequencing switch 33.

The internal circuit diagram is illustrated in FIG. 5. When the sequencing switch 33 (FIG. 1) is in position 1, contacts S1 (on left hand side of diagram) are closed, putting resistance R2 and R3 in series with the voltmeter 30 to provide a voltage check. In sequence switch position 2, contacts S2 are closed, powering the Wheatstone Bridge circuit 41 for temperature measurement. RT-l is the thermistor embodying (FIG. 2) temperature sensitive element 17a located in the lower leg 17 of gun 11. V1 represents a reference potentiometer, varied by surface temperature control knob 34 (FIG. 1) which is calibrated in degrees centigrade and V2 is a small auxiliary preset potentiometer provided to calibrate the circuit by reference to melting ice (0 0). R4 is a suitable resistance for the remaining arm of the bridge. The meter is now connected across points 42, 43 of the Wheatstone Bridge 41 through resistor R1. Balance of this bridge is attained by increasing V1 from its lowest resistance value (highest temperature setting).

With the sequence switch 33 in position '3 contacts S3 are closed, activating the circuit in readiness for heating and ejecting the platen. Thermistor RT2, in series with resistor R5 and potentiometer V3, is the thermistor not shown but located in the heater coil former 19. As thermistor RT2 is heated up, lowering its resistance to complement that of potentiometer V3 which sets, by operation of platen temperature control 35, the desired platen temperature, the voltage signal is taken into a Schmitt trigger circuit consisting of transistors T2, T3 and resistors R7, R8, R9, R10, R11 and V4. If it should prove necessary, a buffer resistor or a buffer transistor stage (neither shown) may transmit this voltage signal. When it is desired to commence the heating cycle the hand switch 32 (shown also in FIGS. 1-3) is closed. At this point transistor T3 of the Schmitt trigger is conducting which forces transistors T7, T8, T9 and power transistors T10, T11 to be also conductive, providing power to heater coils H1, H2 situated on the former 19. Switch SM is provided to take the heater coil H2 out of circuit for the lower platen temperatures. Switch SM is a microswitch coupled to potentiometer V3. As the coil temperature rises, the resistance of thermistor RT2 falls until the Schmitt trigger circuit flips over making T3 non-conducting and T2 conducting. This shuts off transistors T7, T8 and T9 and therefore cuts off the main heating current through T10 and T11. A small leakage current still flows through these heater coils and power transistors, making the heat indicator light 29 glow dimly. As transistor T2 of the Schmitt trigger circuit is now conducting, thyrister TC is switched on through inversion transistor T4 providing power for the solenoid to fire the platen. Afterwards, the hand switch 32 may be opened again thus switching the thyristor TC ofi. Capacitor C1 across the hand switch 32 is provided to quench any are that may form across the hand switch, especially if this is used to switch off the full heater current. With the hand switch off, transistors T5 and T6 conduct to illuminate the reset indicate light 28. As the Schmitt trigger resets due to cooling of the heater coils, T2 is switched off, cancelling the signal of 28.

The value of the components shown in FIG. 4 are shown in Table I:

TABLE I Transistors:

T2 2N3904 T3 2N3904 T4 C83 T 2N3906 T6 0083 T7 2N3906 T8 0C83 T9 0C83 T CQT 1076 T11 CQT 1076 Resistors: Ohms R1 47 R2 8.2K R3 6.8K R4 2.2K R5 3.9K R7 510 R8 110 R9 330 R10 4.3K R11 16K R12 390 R13 300 R14 180 R15 24 R16 24 R17 1.6K R18 47 R19 47 R20 2.7 R21 39 Potentiometers:

1 (log) 5K V2 (lin) 5K V3 (log) 5K V4 (lin) 500 Heater coils:

In use the sequence switch 33 is first moved to position 1 to close contacts S1 so allowing checking of the accumulator voltage by meter 30. The main sequencing switch 33 is then moved to position 2 to close contacts S2 and the gun 11 located against the surface 111 with all three legs 15,, 1 6, 17 firmly in position on the surface 111. The needle of the volt meter will deflect and the surface temperature control knob 34 is then turned anticlockwise until the needle of meter 30 reaches the zero mark. The surface temperature can then be read directly from the calibrations on the control knob 34. The operator then selects a platen with an alloy that melts at a temperature somewhat higher than the measured surface temperature. Conveniently the surface temperature control knob 34 is marked with letters A to P and the platens are marked accordingly. The selected platen is inserted through the splash guard 18 into the aperture 20 in the heating coil former 19. The operator then sets the platen temperature control knob 35 to the appropriate position, and this control knob 35 also may be marked with corresponding letters A to F. (The two control knobs 34, 35 can if required be mechanically coupled.) With the gun 11 located centrally over the spiral pattern marked on surface 111 the heating cycle is then initiated by pressing the trigger 14.

Whilst the heater is working the right-hand heat signal lamp 29 on the top face of the platen gun will glow brightly. As soon as the front face of the platen has reached the correct temperature the gun 11 will without further action from the operator switch off the heaters and eject the platen forwards on to the surface 111. At this point the heat signal lamp 29 will dim appreciably. The trigger 14 should be released as soon as possible afterwards, when the left-hand reset signal lamp 28 will glow. A further platen may only be fired when this lamp 28 has ceased to glow. The sequence switch 33 should be returned to the zero position if a period of more than one minute will elapse before the next platen is to be fired.

I claim:

1. A method of obtaining a replica of a physical pattern on a surface in which the temperature of the surface is measured, a platen coated with a low melting point material whose melting point is higher than the sensed surface temperature is heated to a temperature close to the melting point of the alloy, and is then projected against the surface, such that the alloy receives a complementary or negative replica of the pattern on the surface and hardens.

2. A method of obtaining a replica according to claim 1 in which the material is so selected as to have a similar coefiicient of linear thermal expansion as the material of the surface concerned.

3. Apparatus for taking replicas or impressions of physical patterns on surfaces, comprising a body or housing having incorporated in it an ejection system adapted for ejecting a platen coated with a low melting point material against the surface, heating means incorporated in the housing for heating the platen before it is ejected, detection means incorporated in the housing sensitive to the temperature of the platen, and a firing device incorporated in the housing adapted to energise the ejection system to discharge the platen when a selected platen temperature as detected by the detection means is achieved.

4. Apparatus for taking replicas according to claim 3 in which the housing is provided with locating elements to engage with, and assist in positioning the apparatus relative to, the surface, one of these locating elements including means for sensing the temperature of the surface.

5. Apparatus for taking replicas according to claim 4 including means in the housing for varying the selected platen temperature at which the firing device operates to energise the ejection system.

6. Apparatus for taking replicas according to claim 5 including an automatic sequencing circuit disposed within the housing and arranged firstly to energise the heating means and secondly to energise the firing device when the platen temperature reaches the selected value.

7. Apparatus for taking replicas according to claim 3 utilizing a platen whose surface is fabricated from a material having a similar coefficient of linear thermal expansion as the material of the surface concerned.

8. Apparatus for taking replicas according to claim 7 wherein the material is selected from the group mild steel, aluminium and brass.

9. Apparatus for taking replicas according to claim 7 wherein the material is cast onto a thin layer of bonding metal deposited on the platen.

References Cited UNITED STATES PATENTS 2,676,907 3/1961 Harvey et al. 3,088,200 5/ 1963 Birdsall et al 219-149 X 3,121,014 2/1965 Ducam 219149 JOSEPH V. TRUHE, Primary Examiner P. W. GOWDEY, Assistant Examiner US. Cl. X.R. 219243 

